1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method thereof. The present invention relates to an electronic appliance.
2. Description of the Related Art
In recent years, a technique by which a semiconductor device such as a thin film transistor (also referred to as a TFT) is formed using a semiconductor thin film (having a thickness of approximately several tens of nanometers to several hundreds of nanometers) formed over a substrate having an insulating surface has attracted attention. Thin film transistors are applied to a wide range of electronic devices such as ICs or electro-optical devices, and prompt development of thin film transistors that are to be used as switching elements in image display devices, in particular, is being demanded.
Moreover, there is a trend in an image display device typified by a liquid crystal display device toward a larger screen, e.g., a 60-inch diagonal screen, and further, the development of an image display device has proceeded toward a screen size of a diagonal of 120 inches or more. In addition, a trend in resolution of a screen is toward higher definition, e.g., high-definition (HD) image quality (1366×768) or full high-definition (FHD) image quality (1920×1080), and prompt development of a so-called 4K Digital Cinema display device, which has a resolution of 3840×2048 or 4096×2180, is also demanded. Conventionally, for such a large-sized image display device, a thin film transistor using amorphous silicon has been used as a switching element.
On the other hand, a liquid crystal display device in which the drive frequency is increased in order to improve the display performance of a moving image, a 3D image is displayed, or the like has also been developed. In such a display device, it is difficult to display high-definition moving images in the case of using a thin film transistor using conventional amorphous silicon; therefore, a thin film transistor which responds at high speed and has high field effect mobility is needed and prompt development of a semiconductor device using a semiconductor film with high carrier mobility is demanded. For example, as a semiconductor film with high carrier mobility, which is suitable for manufacturing a large-sized substrate, a thin film transistor using a microcrystalline silicon film is known (Patent Document 1).
FIG. 8 illustrates a structure of the thin film transistor described in Patent Document 1. The thin film transistor includes a gate electrode layer 205, a gate insulating layer 207, a first semiconductor layer 209, a second semiconductor layer 211, impurity semiconductor layers 213a and 213b, a source electrode layer 215a, and a drain electrode layer 215b over a substrate 101. Note that the source electrode layer 215a and the drain electrode layer 215b of the thin film transistor are formed in contact with each side surface of the first semiconductor layer 209 and the second semiconductor layer 211.
By the way, when a thin film transistor is manufactured over a substrate having an insulating surface, as the number of masks used in a photolithography method is decreased, the number of manufacturing steps can be reduced, which leads to cost reduction in manufacturing the thin film transistor.